In well service operations such as well cementing, fracturing and gravel packing, it is necessary to pump fluid slurries containing high loadings of abrasive materials such as and, gravel and/or cement under high pressures into a wellbore. In order to attain the required pressures in pumping such slurries which may be upwards of 20,000 psi in a well fracturing operation with sand loadings of as much as 20 lbs per gallon of fluid, high pressure piston-type of pumps are used. So-called triplex pumps incorporating three pistons or plungers whose pumping strokes effectively balance the discharge pressure are typical of such pressure pumps used in oilfield service operations of this type.
Common triplex pumps for high pressure oilfield service operation typically employ a valving arrangement wherein the valves are located on opposite sides of the pumping chamber along a cross-bore axis which is normal to the stroke axis of the pump plungers. The service life of such a pump and valve arrangement is somewhat limited, however, since there is a high pressure fatigue stress concentration at the juncture of the plunger bore with the transversely oriented valves in the cylinder head.
In an effort to avoid these high stress concentration which limit the service life of such pumps, an in-line valve arrangement such as is used in high pressure water pumps has been tried. With an in-line valve pump, the valve head comprises a central discharge port located along the longitudinal axis of the plunger with annular inlet ports located radially outwardly of the discharge port having valve seating surfaces located both between the discharge port and the annular inlet ports and radially outwardly of the inlet ports.
The discharge port is provided with a check valve which may comprise a ball or disc-type check valve having spring means which, along with back pressure from adjacent cylinders, biases the check valve to the closed position. Similarly, the in-let ports are closed by an annular disc engaging the seating surfaces of the valve head and having a central opening therethrough for passage of fluid into the discharge port when the inlet or suction valve disc is in the closed position. The suction valve disc is biased in the closed position by spring means.
The difficulty with the use of an in-line valve such as used in the pumping of water is the substantial amount of wear on the suction valve and seating surfaces when pumping high pressure slurries containing relatively large amounts of highly abrasive particles such as sand, gravel or cement. The seating surfaces on both the valve head and the valve disc are quickly eroded thereby quickly decreasing the pumping efficiency and the high pressure pumping ability of the pump.
An additional problem with prior in-line valve design with the pumping of high-solids slurries is the build-up of solids within the pumping chamber because the flow characteristics of the disc closure valve do not permit high sweeping efficiency to clear solids from the pump chamber. Increased wear and lower pumping efficiencies result from solids retention within the pump chamber.